An Instrumental Landing System (ILS) approach is a type of precision approach useful in guiding an aircraft to a runway under Instrumental Metrological Conditions (IMC). During an ILS approach, a properly-equipped aircraft utilizes an ILS receiver to receive modulated Glide Slope (G/S) signals defining a G/S beam, which has a fixed orientation relative to the runway surface. After an aircraft intercepts the G/S beam, a pilot controls the aircraft to closely follow the beam, to the extent possible, until touchdown with the runway. To assist in this effort, a pilot may be presented with a cockpit display including G/S guidance symbology visually indicating vertical deviations of the aircraft from the G/S beam during the ILS approach. Additional symbology may also be generated on the cockpit display, such as symbology indicating the lateral deviation of the aircraft from a localizer beam. Such G/S guidance symbology has recently been integrated into Synthetic Vision Primary Flight Displays (SV-PFDs), which feature Synthetic Vision System (SVS) scenes. Generally, the SVS scene of a SV-PFD simulates a glass cockpit view (that is, an unobstructed view from the aircraft cockpit under ideal visibility conditions) and is presented from a particular viewpoint (the SVS viewpoint), which corresponds to the current aircraft position and thus varies in conjunction with movement of the aircraft relative to a fixed frame of reference (the earth).
ILS approaches may be assigned to one of three categorizations: Category (CAT) I, II, and III, with CAT III further subcategorized into CAT III(A), III(B), and III(C) approaches. Of the three primary ILS categorizations, CAT I approaches require the greatest visibility requirements and decision height. Newly-published rules (AC20-SVG/S) issued by the Federal Aviation Administration (FAA) now permit the usage of SV-PFDs during ILS CAT I operations to lower than standard (e.g., 200 feet) approach minimums at properly equipped airfields. Thus, in the case of an SV-PFD including G/S guidance symbology, it becomes highly desirable to minimize or eliminate any mismatch between the SVS viewpoint and the G/S guidance symbology. Conventional cockpit display systems, however, may permit such a mismatch to develop between the SVS viewpoint and the G/S guidance symbology, particularly the vertical G/S deviation symbology. Such mismatches may develop due to vertical errors in the altitude data, which typically dictates the vertical component of the SVS viewpoint. The altitude data may be based upon barometric altimeter readings, Global Positioning System (GPS) altitude data, or a combination thereof.
As noted above, the altitude data utilized to determine the vertical SVS viewpoint can be based upon barometric altimeter readings and/or GPS altitude data. However, both of these altitude data sources can be subject to non-trivial inaccuracies in at least some instances. Inaccuracies in barometric altimeter readings can arise from temperature variations, delayed barosetting reports, pilot set errors, and other factors. Relative to barometric altimeter readings, GPS altitude data tends to be more accurate. However, the altitude data provided by a GPS receiver is also subject to inaccuracies, particularly when the aircraft carrying the GPS receiver operates in non-Space-Based Augmentation System (non-SBAS) environments. Regardless of whether they arise from inaccuracies in barometric altimeter readings or inaccuracies in GPS altitude data, such errors in altitude data can thus result in noticeable mismatches between the SVS vertical viewpoint of an SV-PFD and any G/S guidance symbology further produced on SV-PFD. This can be concerning to a pilot when carrying-out an ILS approach, such as a CAT I ILS approach under zero visibility conditions, while relying upon the G/S vertical deviation symbology for authoritative guidance.